A photodiode is a semiconductor diode, having a pn-junction or p-i-n structure, that functions as a photodetector. A pn-junction is formed by combining p-type and n-type semiconductors together in very close contact, where p-type and n-type denote first and second conductivity types. A p-i-n structure is formed by placing an undoped intrinsic semiconductor region between p-type semiconductor and n-type semiconductor regions.
Typically, a photodiode image sensor includes a reset transistor and a light-sensing region formed by photodiodes. Each photodiode can be formed, e.g., by joining an n-doped region with a p-doped body. During operation, a voltage is applied to the gate terminal of the reset transistor such that the reset transistor is switched on and a capacitor at the pn-junction is charged up. When this capacitor is charged to a certain potential level, the reset transistor is switched off such that the photodiode is reverse biased, leading to the formation of a depletion region. When a photon of sufficient energy strikes the photodiode, it excites an electron thereby creating a mobile electron and a positively charged electron hole. If the absorption occurs in the pn-junction's depletion region, or one diffusion length away from it, these carriers are swept from the pn-junction by the built-in electric field of the depletion region, producing a photocurrent. That is, the electrons migrate towards the n-doped region such that the electric potential in the n-doped region will drop. On the other hand, the holes move away from the depletion region towards the p-doped body.
Typically, the pn-junction is located in a certain depth from a semiconductor main surface leading to a certain spectral selectivity of the photodiode structure. Light of short wavelength is attenuated more than light of a longer wavelength when entering the semiconductor through the main surface. Therefore, a photodiode can be made sensitive for short-wavelength light if the depletion region is formed in a depth close to the semiconductor surface and, on the other hand, a photodiode structure can be made sensitive to longer-wavelength light if the depletion region is arranged in a greater depth from the semiconductor main surface.
It would be desirable to have a photodiode structure fabricated in typical CMOS processing steps, which is sensitive to both light of short and longer wavelengths.